Permanently change hostname of Debian Squeeze virtual server

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I have a virtual server running Debian Squeeze. I can change the hostname by running the hostname new.host.name command or by editing /etc/hostname and then running /etc/init.d/hostname.sh, but after a reboot the hostname will be reset to the preconfigured hostname chosen by the server-hoster. I do not know which virtualization solution is used and I do not have access to an admin interface to change the hostname.
How can I change the hostname with a script each time the server boots?
(HowTos)